The present invention relates to a superconducting magnet system which has a superconducting coil and a persistent current switch and more particularly to a superconducting magnet system which has quench protection circuits for the superconducting coil and the persistent current switch.
Superconducting coils capable of flowing a large current are often used in magnets required to generate strong magnetic fields, as in NMR (nuclear magnetic resonance) equipment. To use superconducting coils to generate strong magnetic fields that are stable at all times requires flowing an electric current in a persistent current mode. To establish the persistent current mode, a persistent current switch (PCS) is connected in parallel with the superconducting coils.
The superconducting coils, however, have a problem of quench. In the event of a quench, if a protection circuit is not provided, energy stored in the superconducting coils is all consumed by the superconducting coils themselves that have transitioned to a normal conducting state. As a result, the superconducting coils may be burned out or their composition change. Further, since the persistent current switch is also a superconducting device, it must be provided with a protection circuit to protect against quenches, as with the superconducting coils. Therefore, superconducting magnets need to be connected with quench protection circuits.
FIG. 4 shows one example of superconducting magnet circuits made up of superconducting coils, persistent current switches and quench protection circuits. Three series-connected superconducting coils 10, 12, 14, a persistent current switch 16 and a protective resistor 18 are parallelly connected. These are also connected in parallel with a power supply (not shown) through current leads 20, 22 for excitation and deexcitation. This magnet circuit also has a persistent current switch 24 for commutation, a heater drive circuit 26 for the persistent current switch, and resistive heaters 28, 30, 32.
In the circuit shown in FIG. 4, in the event that the superconducting coils 10 to 14 quench, the energy stored in these coils is mostly consumed by the protective resistor 18 by making the resistance of the protective resistor 18 sufficiently smaller than the off-state resistance of the superconducting coils 10 to 14. Also, when the persistent current switch 16 quenches, most of the energy is consumed by the protective resistor 18 by making the off-state resistance of the persistent current switch 16 sufficiently larger than the resistance of the protective resistor 18.
If the off-state resistance of the persistent current switch 16 is large, a time constant during deexcitation becomes large, which means that it takes longer for deexcitation. In addition, the current flows through both the persistent current switch 16 and the protective resistor 18. This increases the amount of heat produced, so a diode or thyristor is being considered in place of the protective resistor. These techniques are described in JP-A-7-27815.